On one of my jobs, we have a huge drift of V, bonariensis, which just self seeds itself each year, the ground they are in is free draining, the ground was at one point quite heavy, but with generous helpings of compost and grit, it has become very good for the Verbena, that was many years back now, we never feed them just put a good layering of composted leaves over the area at the back end and lightly work it in, they always put on a grand show.

Thanks Julien! I hope mine survive long enough for this treatment!